This invention relates to the use of a composite laminate as a membrane in composting systems. More particularly, the invention relates to a composting system having a membrane that is waterproof, air and moisture vapor permeable, and includes a grid layer for controlling the return of condensate to a compost pile. Treatment of solid fermentable organic wastes such as municipal sludge, household organic waste, agricultural waste, and other organic wastes including hydrocarbon-contaminated soils, has been achieved using various composting methods. For example, the waste materials can be spread in windrows or boxes where they ferment and produce soil or growth substrate improvement products. When the fermentation is conducted in the presence of air (aerobic fermentation) oxygen is consumed and the fermentation process produces carbon dioxide, water, and heat, resulting in an increase in temperature of the composted material. In traditional composting, the supply of oxygen is replenished by turning the windrows from time to time. The composting cycle time (time it takes for the organic matter to be degraded by microorganisms) is relatively long using this method, typically about six months or more. In addition, the compost pile takes up a considerable amount of land space.
Various methods have been used to accelerate the composting process. For example, forced aeration by blowing or sucking air into the compost windrow has been used to accelerate the composting process. Microbial activity is monitored by measuring the temperature and/or oxygen saturation at different points in the windrow. While controlled aeration has resulted in a significant reduction in composting cycle time, air channeling through the compost can be a problem, resulting in anaerobic areas throughout the windrow and unacceptably non-homogenous soil or compost. In addition, the exhaust from a forced aeration process carries with it odorous substances as well as bacteria and microorganisms which can be potentially harmful to humans and can create an unacceptable environmental nuisance to the neighboring communities.
In order to overcome these problems, breathable membranes have been introduced to cover the compost windrows or boxes in forced air systems. These membranes allow sufficient air and water vapor to pass through them, thus insuring the supply of oxygen to the microorganisms and the removal of excess heat and water vapor from the compost. In addition, the membranes are impermeable to rainwater, thus preventing the compost pile from becoming too wet. When the compost is protected from rainwater, the channeling problems discussed above are greatly reduced and oxygen mass transfer within the pores and micropores of the compost pile is enhanced. In addition, the membranes can be useful for reducing the escape of odorous molecules, bacteria, and fungi. Examples of membranes that have been used in composting systems include a microporous polyurethane sandwiched between two woven or non-woven materials, as described in German patent DE 4231414 to Ploucquet GmbH and a material supplied by W. L. Gore and Associates which has an expanded porous polytetrafluoroethylene layer sandwiched between two woven polyester materials.
Fixed composting systems in which the composting process is carried out in fixed enclosures are also known. Such systems generally comprise an enclosure having three side walls, a door for adding or removing organic waste or compost to and from the structure, and a slanted roof. The roof may be openable, such as those provided by BIODEGMA GmbH of Stuttgart, Germany, so as to permit a front-end-loader to deliver waste material or remove fresh compost. Air is supplied through the floor into the base of the compost pile. Membranes such as those described above can be installed in the roof of fixed composting enclosures. In these systems, a portion of the water vapor which is generated during the composting process condenses on the inside surface of the membrane to form water droplets which are returned to the compost layer below when they detach from the membrane surface. When conventional membranes are used, a significant percentage of the water droplets will travel downwardly along the surface of the membrane without detaching until the droplets impact upon one of the side walls of the enclosure. The water then runs down the side wall and is concentrated at the edges of the compost pile, resulting in a non-uniform moisture distribution in the compost and causing wet and dry areas which are undesirable for the reasons mentioned above.
There is provided by the present invention a composting system comprising an enclosure for containing organic matter undergoing composting. The enclosure has an access through which organic waste can be introduced into the enclosure and through which composted matter can be removed from the enclosure. The enclosure has a sloped roof comprised of a composite laminate which includes an air and moisture permeable membrane having an inside surface that faces into said enclosure. The inside surface of the membrane is adhered to an open grid layer. The composting system may further comprise means for introducing fresh air into organic matter being composted within the enclosure.